The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 950 and the like. That is, these standards provide that at least three insulating layers are to be formed between primary and secondary windings in a winding, subject that an enamel film covering a conductor of a winding is not admitted as an insulating layer, and that the thickness of an insulating layer is to be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, is to be 5 mm or more, and that the transformer withstands a voltage of 3,000 V applied between the primary and secondary sides for one minute or more, and the like.
According to the standards, a conventional transformer has a structure like that illustrated in the cross-section shown in FIG. 2. In the structure, an enameled primary windings 24 (a conductor: 24a, an enamel coating: 24b) is wound around a bobbin 22 on a ferrite core 21, in such a manner that insulating barriers 23, to secure the creeping distance, are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 25 (a first layer 25c, a second layer 25b, and a third layer 25a) is wound for at least three turns on the primary winding 24; additional insulating barriers 23, to secure the creeping distance, are arranged on the insulating tape, and an enameled secondary winding 26 (a conductor: 26a, an enamel coating: 26b) is then wound around the insulating tape. Further, an insulating tape 27 is wound thereon.
Recently, a transformer having a construction that includes neither the insulating barriers 23 nor the insulating tape layer 25, as shown in FIG. 1, has started to be used in place of the transformer having the construction shown in FIG. 2. The transformer shown in FIG. 1 has an advantage over that shown in FIG. 2, in that it can be reduced in overall size and dispenses with the winding operation for the insulating tape.
In respect to the transformer shown in FIG. 1, the primary windings (or the secondary windings) have three insulating layers, an innermost layer 14b (or an innermost layer 16b), an intermediate layer 14c (or an intermediate layer 16c), and an outermost layer 14d (or an outermost layer 16d), formed on the outer peripheral surface on a conductor 14a (or a conductor 16a).
A winding in which an insulating tape is first wound around a conductor to form a first insulating layer (an innermost layer) thereon, and is further wound to form a second insulating layer (an intermediate layer) and a third insulating layer (an outermost layer) in succession, so as to form three insulating layers that are separable from one another, is known. Further, in place of insulating tapes, it is known that fluororesins are sequentially extruded to cover the outer periphery of a conductor to entirely form three insulating layers (see, for example, JU-A-3-56112 (“JU-A” means unexamined published Japanese utility model application)).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance and high-frequency characteristic. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the external appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the electric wire becomes high.
To solve such problems, a multilayer insulated wire has been put into practical use, which is obtained by extruding denatured polyester resins the crystallization of each of which is controlled and a reduction in molecular weight of each of which is suppressed as first and second insulating layers and a polyamide resin as a third insulating layer to cover the outer periphery of a conductor (see, for example, U.S. Pat. No. 5,606,152, JP-A-6-223634 and the like (“JP-A” means unexamined published Japanese patent application)). In association with recent miniaturization of electrical and electric equipment, an influence of heat generation on the equipment has been concerned, so a multilayer insulated wire with improved heat resistance has been proposed, which is obtained by extruding a polyethersulfone resin as an inner layer and a polyamide resin as an outermost layer to cover the outer periphery of a conductor (see, for example, JP-A-10-134642).
However, in association with further miniaturization of electrical and electric equipment, it has been required that an insulated wire involve excellent solvent properties to cope with a solvent treatment after wiring processing in terms of handling, and involve improved heat resistance. At present, no insulated wires satisfying all of those properties have been obtained.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.